That's.... a really tough one.
Gen I comes into a good amount of hate, but it's also probably the most beloved by the fanbase. Besides, a lot of the hate seems to stem more from a reaction to overly vocal fans rather than from the generation itself.
Gen II.... I'd almost call that one 50-50. For every person you find who enjoys the Johto games, there's someone who dislikes them.
Gen III? Kind of the same deal as Johto, except the love and hate are both more intense.
Gen IV and specifically Sinnoh seems like a good candidate. There are people who really love it, but it seems you're more likely to run into people who didn't care about it or even hated it. However, it did manage to appease many of the people who hated Sinnoh with the Johto remakes.
As for Gen V, despite the huge fights about it I've seen on the internet, I think it's almost in the same category as Gen I. There's plenty of hate, but it's also beloved by a huge number of fans. It's pretty much become neo-Gen I. It's not the original, but it takes a lot of what made the original special while adding its own elements to it.
I guess in the end, I have to go with Gen IV. Which makes me rather sad because Gen IV is what really drew me deeply into Pokemon, but it seems to me that Sinnoh is the least liked of all the regions and also the one that is widely criticized without being defended. Make a negative remark about any other generation that you please, and you are very likely to get into a heated discussion of some sort. Gen IV? No one really seems to give a flying Grumpig.